Muscular Dystrophy

Muscular dystrophy (DIS-tro-fee) is a group of inherited disorders in which there is a gradual deterioration and weakening of muscles in the body.

The term muscular dystrophy actually refers to a group of disorders that affect different muscles in the body and that may range from mild to severe. Although all of the muscular dystrophies are known to be caused by genes * , as of 2016, the way in which the genes produce the disorder was only partly understood.

The muscles that become weakened in muscular dystrophy are mostly the voluntary muscles, those that people can control when they want to move different parts of their body. Another name for voluntary muscle is skeletal muscle. The weakness is usually symmetrical; that is, it occurs more or less equally on both sides of the body. Muscle deterioration is also progressive, meaning it gradually increases over time. There is no pain connected with the weakening condition, although there may be some cramps and stiffness. Cognitive disabilities or developmental delays sometimes accompany this condition.

All types of muscular dystrophy are uncommon. Many people have come to know about muscular dystrophy through the efforts of American comedian and actor Jerry Lewis, who appeared on television on behalf of the Muscular Dystrophy Association. Similar media appeals, such as the Télethon in France, have raised public awareness about the disease in other countries.

The most common and severe form of the disorder, called Duchenne (du-SHEN) muscular dystrophy, affected approximately 1 in every 3,500 to 6,000 male births each year in the United States as of 2013. (Although girls can carry the gene that causes this disease, they usually have no symptoms.) Muscular dystrophy affects all races, ethnic groups, and social classes equally.

Whether individuals have a form of muscular dystrophy has already been determined through hereditary factors by the time they are born (even though its effects are not apparent at birth). Because it is an inherited disorder, the disease is not in any way contagious, and it is not caused by anything that might happen in everyday life.

The defective gene that causes Duchenne muscular dystrophy is located on the X chromosome * . Therefore, the disorder is said to be sex-linked. It also is recessive, which means that females who inherit the defective gene will not usually develop the disease (because they need to have the defective gene on both X chromosomes) but are carriers of it. They can pass it on to the next generation. Affected males always inherit the gene for Duchenne muscular dystrophy from their mothers, and each male born to a woman who is a carrier for the disease has a 50-50 chance of inheriting the gene.

Myotonic (mi-o-TON-ik) dystrophy, an adult form of the disorder, is inherited as an autosomal dominant gene. Autosomal means that the gene that causes the disorder is not located on a sex chromosome. Dominant means that just one defective gene in a pair is able to produce the disease. Males and females are equally affected by the faulty gene, and both can transmit the disease to their children.

Other types of muscular dystrophy follow various different patterns of inheritance, and they can affect both children and adults.

Seen under a microscope, muscles are made up of bundles of individual muscle fibers. Under greater magnification, each fiber is seen to have an outer membrane. Dystrophin is believed to be responsible for maintaining the structure of the muscle fiber membrane. Without it, the membrane tends to fall apart and become “leaky.” One of the substances that leaks out from inside the muscle fibers is creatine kinase (KREE-a-tin KI-naze), which is needed for the chemical reactions that produce energy for muscle contraction.

In other types of muscular dystrophy, the defect lies in an abnormality in another substance closely associated in function with dystrophin.

There are several forms of muscular dystrophy. Some experts have listed as many as 30 types. Classification of types is based on a person's age at the start of the disease, the location of the muscles affected, the rate at which the disorder progresses, and the pattern of inheritance. The following are signs and symptoms of the more common and well-defined forms.

The signs of Duchenne muscular dystrophy may not be noticed until ages three to seven, when the young boy is likely to start having difficulty walking. (Because of the way it is inherited, only boys usually have Duchenne muscular dystrophy.) Another characteristic sign is that the calf muscles, although becoming weaker, are enlarged partly because of accumulating deposits of fat in them.

In this form, the signs and symptoms are the same as those of Duchenne muscular dystrophy, but they begin later in life and progress more slowly. The same gene that causes the Duchenne form is responsible, but its defects are less damaging to muscle. Most affected men must eventually use a wheelchair, but some do not, and many live past middle age.

Myotonic dystrophy usually develops in adulthood, but it may occur as early as infancy. Males and females are affected equally. The characteristic symptom is myotonia, a delayed relaxation of muscle after it has contracted. Weakness commonly occurs in the muscles of the hands, face, neck, forearms, and lower legs.

Unlike other dystrophies, myotonic dystrophy may involve parts of the body other than the voluntary muscles. For example, the heart rate may be abnormally slowed. Cataracts may develop in the eyes. As in other dystrophies, cognitive disabilities sometimes accompany the condition. Nonetheless, people with mild forms of myotonic dystrophy may have relatively normal lives and survive beyond middle age.

Facioscapulo-humeral (fay-she-o-skap-yoo-lo-HU-me-ral) muscular dystrophy gets its long name from the fact that it weakens the muscles of the face, shoulders (the scapula is the shoulder blade), and upper arms (the humerus is the upper arm bone). Facial expression is altered, and the shoulders tend to droop. Both males and females are affected, and the progression of symptoms is usually slow. Symptoms often begin to appear between the ages of 10 and 40.

Muscle weakness in this form occurs mostly around the hips and shoulders (limb girdles). Symptoms may eventually extend to other muscles. Often, however, worsening of the condition is slow. Both sexes are affected, and symptoms may begin in late childhood or early adult life.

A doctor may suspect that someone has muscular dystrophy just by observing certain signs of muscle weakness. If someone in the patient's immediate family is known to have the disorder, the diagnosis becomes clearer.

To confirm the diagnosis, a blood test may be performed to look for high levels of creatine kinase, mentioned previously as a sign of Duchenne muscular dystrophy. Blood samples may also be analyzed for defective genes and can help determine the specific type of the disorder. Other tests can measure muscle activity, and a muscle biopsy, in which a tiny sample of muscle tissue is removed to be examined under the microscope, can show specific abnormalities.

As of 2016 there was no cure for muscular dystrophy. Sometimes treatment with steroids was able to slow progress of muscular weakness. Treatment was aimed at maintaining general good health and mobility for as long as possible. An important goal is preventing scoliosis and often fatal lung infection. Moderate exercise, physical therapy, the use of braces, and sometimes surgery can assist walking.

Genetic counselors, using medical tests and family history, can help prospective parents make informed decisions about having children. The disease can be diagnosed in a baby before it is born; this is called prenatal testing.

Meanwhile, medical scientists in the laboratory have been working on ways to attack the cause of muscular dystrophy directly through a technique called gene therapy.

See also Genetic Diseases: Overview • Scoliosis

* genes (JEENS) are chemical structures composed of deoxyribonucleic acid (DNA) that help determine a person's body structure and physical characteristics such as hair or eye color. Inherited from a person's parents, genes are contained in the chromosomes found in the body's cells.

* chromosome (KRO-mo-zom) is a unit or strand of DNA, the chemical substance that contains the genetic code to build and maintain a living being. Humans have 23 pairs of chromosomes, for a total of 46.